Suppot for root URL config option & various minor doc tweaks

I intend to run Rattic under a location, rather than root of the vhost - eg: http://localhost/rattic/ rather than http://localhost/.

This pull request includes a new configuration option which can be provided with a URL root for your web application. Existing users of Rattic don't have to change any config, it's purely an optional addition.

Also sets the API URL, which should go some way towards fixing issue 122. :-)